Testing of electrical circuits



Nov. 2, 1937. K. J. R. WILKINSON 2,098,212

TESTING OF ELECTRICAL CIRCUITS Original Filed Nov. 21, 1936 4 SURGE CURRENT SOURCE I :I z 5 E 5 C/JTHODE RAY 05C/LLOGRAPH Fig.7..

II I x i -/a Inventor: Kenneth James Rawcliffe Wilkinson,

H is Attorneg.

Patented Nov. 2, 1937 TESTING OF ELECTRICAL CIRCUITS Kenneth J. R; Wilkinson, Rugby, England, as signor to General Electric Company, a corporatlon of New-York Original application N 9vember 21, 1936, Serial No. 111,990. Divided and this application April 29, 1937, Serial No. 139,792.

In Great Britain October 31, 1935 6 Claims.

The present application-is a division of the copending application of Henry Trencham and Kenneth James Rawclifie Wilkinson, filed November 21, 1936, Serial No. 111,990.

which has zero value until its commencement when time t is zero.

In carrying out the invention for determining the restriking voltage duty of circuit breakers,

This invention relates to the testing of electric the testing surge current may be applied to the 5 circuits by the application thereto of surge cursystem to be tested across the open terminals of rents especially with a view to the determination a circuit breaker and the resulting voltage surge of voltage behaviour. Where complex circuits are is then measured across the breaker terminals .by involved, it is frequently diflicult to obtain the means of a high speed cathode ray oscillograph.

10 voltage reaction of such circuits to an applied In alternating current circuit breakers the 10 surge current of a defined wave form and one obinterruption of the circuit takes place at or close Ject oi the invention is to enable this to be done to the time when the current passes through a with simple apparatus without the necessity for zero value and it is known that the performance determining circuit constants and then using of a circuit breaker is very much influenced by the complex calculation. rate at which the restriking voltage rises across 15 The method of testing an electric circuit with its terminals at this time of current zero. For a view to the determination of its electrical charsuccessful circuit interruption, insulation value acteristic according to the invention, consists in must be built up between the breaker contacts injecting into the circuit re-current surge curat a rate which will prevent the restriking voltage rents of controlled wave form and indicating the from bridging the arcing space and restarting the 20 resultant voltage across points in the circuit by flow of current. means of a cathode ray oscillograph, the beam It is important that the designers of circuit control of which is similarly recurrent and so breakers should know the nature of the re-strikarranged that the resulting voltage across the ing voltage which occurs at any point where a points appears as a standing wave on the screen breaker is installed and the present invention 25 oi the oscillograph. More particularly the inenables such rate of rise to be ascertained. vention relates to the testing of alternating cur- Hitherto means employed for determining the rent circuits to determine the restrlking voltage rate of rise of restriking voltage have been limited which a circuit breaker has to withstand when to cathode ray oscillograms taken when interinstalled in a given position in an electric cirrupting fault current, usually in aspecially equip- 30 cuit. ped testing station with a particular form of test- The relevant duration of restriking voltage ing circuit or, alternatively, to calculation, for surges which occur on any system on the openthe making of which there must be known the ing of a circuit breaker is normally not greater values of circuit constants, i. e., capacitances and than about 10 per cent of the normal cycle (asinductances associated with the system at the 35 suming 50 cycles). point where the circuit breaker is located.

A further object of the invention is to obtain The former method is not economically feasible a recurrent testing current surge which closely for obtaining information on system conditions represents in shape during this period that of the in general and the latter, apart from the tediouscurrent which is, in effect, to be opened by the ness of making the calculation, depends on the 40 circuit breaker, l. e. a sin w t when a is the crest accuracy with which lumped constants can be value of the current w is 21r times the frequency, measured or assumed for each particular case. and the expression has zero value until its com- In accordance with the present invention it is mencement when time t is zero. proposed and preferred to measure directly the The reason for this course s en y e pr restriking voltage surge at any point on a system 45 ciple of superposition from wh h i ws that at which a circuit breaker is installed or is to be if a voltage resu t om suppressing Current in installed, by injecting recurrent surge currents a given ir uit. then an qual v a is tai into the system at the open terminals of the when current of the sa e p t d and shape breaker, and measuring the ,resulting voltage but of pp e Sign s injected into that Circuit across the breaker terminals by means of a cath- 50 and at the same terminals. ode ray oscillograph controlled as hereinbeiore The invention further consists in the use for Specified, obtaining this recurrent testing surge current of The recurrent surge currents may be obtained a mutual inductance to the primary of which is b applying t th primary of t mutual applied a recurring voltage of a shape b cos w t ductance a section of an alternating current volt- 55 age of sine wave form, the desired section being selected by a grid-controlled arc discharge device which is triggered" into operation at each instant of the cycle of the applied alternating current when the voltage is at its crest value.

This triggering may be eflected by means of a suitable combination of capacitance and resistance included in the grid circuit of the discharge device.

The accompanying drawing shows in Fig. 1 a method whereby the invention may be used for testing circuits for determining the duty of circuit breakers, while Fig. 2 shows a circuit arrangement embodying a cathode ray tube, for generating and examining the recurrent surge currents and voltage respectively.

In Hg. 1, I represents a circuit to be tested for determining the duty of a circuit breaker l, the circuit including a generator 3 and being earthed at each end. Four indicates a source of recurrent surge currents being connected across the open terminals of the breaker 2. Five designates a cathode ray oscillograph connected across the open terminals of the circuit breaker 2 and controlled so that the recurring surge voltages across the breaker terminals appear as a standing wave on the screen thereof.

Turning now to a consideration of Fig. 2, we have at I the primary winding of a transformer which is supplied with alternating current of commercial frequency and voltage, say 250 volts 50 cycles; the transformer has three secondary windings I, I, and I. of which 8 controls the scanning of the beam of a cathode ray oscillograph il along the time axis, 9 supplies control potentials for the beam of the oscillograph, i0 supplies an alternating current of controlled wave form to the primary of a mutual inductance I 2 indicated as a transformer, the secondary winding I I of which is connected to inject the recurrent surge currents into the circuit under test.

The secondary winding I is connected to the scanning coil is of the oscillograph so as to deflect the beam produced by the electron gun I 4 of known construction across the screen It ilfty times per second. The magnetic axis of the coil I8 is indicated by the interrupted line a.

One end of the secondary winding 9 is connected to the second accelerator of the oscillograph and earthed. The other end of winding I is connected to the beam control electrode of the oscillograph and the winding 0 is shunted by resistors lb, II and H the junction between II and I. being connected to the control electrode of the oscillograph, while the junction between it and i1 is connected to the anode or first accelerator of the oscillograph. As the accelerating electrodes are periodically made negative with respect to the cathode, the beam is suppressed during alternate half-cycles of the applied alternating current so that the beam only traces a path on the screen during the remaining halfcycles.

The secondary winding I l is connected to energise the primary winding of the mutual inductance it through a controlled arc discharge device it which is periodically rendered conductive by the control of the control grid through a resistance condenser combination 20, 2| connected across winding ID. The values of condenser 2i and of resistance 20 are so chosen that where t is measured from zero, and the resulting output current of the secondary winding of I! is a surge current of form a sin to t where t is measured from zero.

The mutual inductance II is designed to have no interwinding capacity by providing effective screening between the primary and secondary windings, e. g. by providing metal foil coverings over the primary winding, care being taken not to allow a complete short-circuited turn. If more than one secondary voltage is desired the primary and/or secondary may be tapped, though it is preferred to use interchangeable coils.

The resulting surge voltage across the open points of the circuit under test is applied to the beam deflecting plates ,of the oscillcsr'aph preferably through an amplifler II. Owing to the supply being taken from a common transformer thedeflectingvoltageacrosstheplatesisso timed with respect to the time deflection of the beam obtained from coil I. that recurring deflections of the beam are superposed and the resulting trace on the screen appears as a standing wave thereon. and this trace may be readily examined and photographed if desired.

whatlclaimasnewanddesiretosecureby Letters Patent of the United States is:

1. In combination. a circuit to be tested having a break therein, said circuit normally being closed and including a current supply source, means for applying at said break a current having a wave form corresponding to that of current flowing in the circuit when unbroken, and means for observing the resultant wave form of the voltage which appears across said break.

2. In combination, a circuit to be tested having a break therein, said circuit normally being closed and including a current supply source. means for applying at said break recurring surges of current having a wave form corresponding to that of current flowing in the circuit when unbroken, and means for observing the resultant wave form of the voltage which appears acrothe break.

3. A method of testing an electrical circuit including a source of current to determine the voltage transient set up in said circuit by interruption of the normal current flowing therein which comprises introducing into said circuit at a break therein recurring current surgu of the same wave form as the normal current flowing in said circuit and observing the resultant wave form of the voltage which appears across said break.

4. A method of testing an electrical circuit to determine the voltage transient set up in said circuit by the interruption of current of predetermined wave form flowing therein which comprises introducing into said circuit at a break therein a surge current having said predetermined wave form and observing the resultant wave form of the voltage which appears across said break.

5. In combination, a circuit to be tested having a break therein, said circuit normally being closed and including a current supply source, a mutual inductance having a primary and a secondary winding, means for applying a current surge to said primary having a wave form corresponding to the wave form of the current flowing in said circuit when unbroken, said secondary being adapted to be connected across a break in said circuit to be tested, and means for producing an indication of the resultant wave form of the voltage which appears across the break.

wave form of the current flowing in said circuit when unbroken, and means for observing the resultant wave form of the voltage which appears across the break.

KENNETH J. R. WILKINSON. 

